Reflection type photomask and reflection type photolithography method comprising a concavo-convex surface

ABSTRACT

A reflection type photomask includes a substrate, and a reflecting surface formed on the substrate and including a first region and a second region which have a relative height difference. Due to the concavo-convex structure of the reflecting surface, a light reflected from the first region and a light reflected from the second region have a predetermined phase difference which may be used effectively to form a pattern on a photoresist layer.

This is a division of application Ser. No. 07/667,934 filed Mar. 12,1991 U.S. Pat. No. 5,190,536.

BACKGROUND OF THE INVENTION

The present invention generally relates to photomasks andphotolithography methods using photomasks, and more particularly to areflection type photomask and a reflection type photolithography methodwhich uses the reflection type photomask.

In the field of semiconductor devices, there is a demand to form finerpatterns in order to further reduce the size of integrated circuits. Inorder to form finer patterns, it is necessary to improve the resolution.The resolution is inversely proportional to the numerical aperture andis proportional to the wavelength of the light which is used for theexposure. However, there is a limit to increasing the numericalaperture, and thus, it is necessary to reduce the wavelength of thelight in order to improve the resolution. But on the other hand, variousrestrictions are introduced when the wavelength of the light is reduced.

According to the conventional photolithography technique, a transmissiontype mask is transferred on a photoresist layer by use of a transmissiontype lens when forming a pattern. But when the wavelength of the lightis reduced in order to improve the resolution, various restrictions areintroduced by the material used for the lens and the substrate materialof the mask. That is, when the wavelength of the light is graduallyreduced in an ultraviolet region, the electron transition region of thematerial starts and the light transmittance decreases rapidly. Inaddition, color center or the like is generated by impurities andlattice defect. Furthermore, it is virtually impossible to use asuitable light transmitting material in the far ultraviolet region.

Therefore, according to the conventional transmission typephotolithography technique, it is in actual practice impossible uselight having a wavelength of approximately 180 nm or less due to therestrictions posed by the transparent material. As a result, theresolution limit of the conventional transmission type photolithographytechnique is approximately 0.2 to 0.25 μm.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful reflection type photomask and reflection typephotolithography method, in which the problems described above areeliminated.

Another and more specific object of the present invention is to providea reflection type photomask comprising a substrate, and a reflectingsurface formed on the substrate and including a first region and asecond region which have a relative height difference, so that a lightreflected from the first region and a light reflected from the secondregion have a predetermined phase difference caused by a difference inlengths of optical paths of the lights which reflect at the first andsecond regions. According to the photomask of the present invention, thelight from the light source is reflected by the reflecting surface, andthe wavelength of the light used will not be limited by thetransmittance of the material used for the photomask. Hence, it ispossible to realize a photolithography using a light having a wavelengthof 180 nm or less, and a pattern having a size of 0.2 μm or less can beformed accurately by use of the photomask.

Still another object of the present invention is to provide aphotolithography method comprising the steps of irradiating a light froma light source on a reflection type photomask which has a reflectingsurface which includes a first region and a second region which have arelative height difference so that a light reflected from the firstregion and a light reflected from the second region have a predeterminedphase difference caused by a difference in lengths of optical paths ofthe lights which reflect at the first and second regions, imaging thelights reflected from the first and second regions of the photomask ontoa photoresist layer which is formed on a wafer by use of an opticalsystem so as to develop a pattern on the photoresist layer, anddeveloping the pattern on the photoresist layer depending on a lightintensity of the light imaged thereon. According to the photolithographymethod of the present invention, the phase of the reflected light isshifted using the concavo-convex reflecting surface of the photomask.Hence, the pattern is formed on the photoresist layer using only thereflected light from the photomask, and the wavelength of the light usedwill not be limited by the transmittance of the material used for thephotomask. Hence, it is possible to realize a photolithography using alight having a wavelength of 180 nm or less, and a pattern having a sizeof 0.2 μm or less can be formed accurately by use of the photomask.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining an embodiment of a reflection typephotolithography method according to the present invention;

FIGS. 2A and 2B respectively show an amplitude of reflected light andintensity of reflected light for explaining a reflection type photomaskshown in FIG. 1;

FIGS. 3A and 3B respectively are a cross sectional view and reflectedlight intensity distribution of a first embodiment of the reflectiontype photomask according to the present invention;

FIGS. 4A and 4B respectively are cross sectional views for explaining acase where a metal substrate is used for the photomask and a case wherea quartz substrate is used for the photomask;

FIGS. 5A and 5B respectively are a cross sectional view and reflectedlight intensity distribution of a second embodiment of the reflectiontype photomask according to the present invention;

FIGS. 6A and 6B respectively are a cross sectional view and reflectedlight intensity distribution of a third embodiment of the reflectiontype photomask according to the present invention;

FIGS. 7A and 7B respectively are a cross sectional view and reflectedlight intensity distribution of a fourth embodiment of the reflectiontype photomask according to the present invention;

FIGS. 8A and 8B respectively are a cross sectional view and reflectedlight intensity distribution of a fifth embodiment of the reflectiontype photomask according to the present invention;

FIGS. 9A and 9B respectively are cross sectional views for explainingsixth and seventh embodiments of the reflection type photomask accordingto the present invention in which the reflection is reduced;

FIGS. 10A and 10B respectively are a plan view and a cross sectionalview of a photomask for forming a contact hole;

FIG. 10C is a plan view showing a developed pattern obtained by use ofthe photomask shown in FIGS. 9A and 9B;

FIGS. 11A and 11B respectively are a plan view and a cross sectionalview of a photomask for forming a metal interconnection pattern;

FIG. 11C is a plan view showing a developed pattern obtained by use ofthe photomask shown in FIGS. 11A and 11B; and

FIGS. 12A and 12B are cross sectional views for explaining the effectsof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a description will be given of an operating principle of anembodiment of a reflection type photolithography method according to thepresent invention, by referring to FIG. 1. In FIG. l, a reflection typephotomask 1 has a bottom surface which forms a reflecting surface. Thereflecting surface includes a first phase region 2 and a second phaseregion 3. The first phase region 2 is a concave part relative to thesecond phase region 3, or the second phase region 3 is a convex partrelative to the first phase region 2.

A light emitted from a light source 7 is reflected by an illuminationmirror 6, and a reflected light 11 is irradiated on the photomask 1.Because the photomask 1 has the concavo-convex reflecting surface, areflected light 12 from the reflecting surface includes lights having aphase difference of π radians, for example, depending on theconcavo-convex structure. The reflected light 12 is reflected andconverged by an imaging mirror 5 and is imaged on a photoresist layer 8which is formed on a substrate (or wafer) 4. The substrate 4 is securedby a support means 13 such as a vacuum chuck. For example, the substrate4 is made of a semiconductor.

Because of the concavo-convex structure of the reflecting surface, alight which is reflected at the first phase region 2 propagates throughan optical path which is longer than a light which is reflected at thesecond phase region 3. In addition the light reflected at the firstphase region has a phase which is delayed compared to the phase of thelight reflected at the second phase region 3.

When the light reflected at the first phase region 2 and the lightreflected at the second phase region 3 have a phase difference π, anamplitude Er of the reflected light has an inverted phase betweenregions II and III in FIG. 2A. On the other hand, when a non-reflectingregion having a low reflectivity is provided on the reflecting surfaceof the photomask 1, an absolute value of the amplitude Er of thereflected light becomes small as indicated by a region I in FIG. 2A.

When the photoresist layer 8 is exposed, the photoresist layer 8 sensesan intensity Ir of the reflected light which is irradiated thereon. Theintensity Ir of the reflected light is proportional to the square of theamplitude Er of the reflected light, and thus, the light intensitydistribution becomes as shown in FIG. 2B. In other words, the lightintensity Ir is approximately the same in the regions II and III, butthe light intensity Ir is zero at the boundary of the two regions II andIII. The light intensity Ir at the boundary decreases to an intermediatevalue if the phase difference between the lights is less than π.

Accordingly, when lights having mutually different phases are diffractedto mutually interfere, it is possible to reduce the intensity of thereflected light which is irradiated on the photoresist layer 8 on thesubstrate 4 depending on the amplitude and phase of the lights. Whenlights having identical amplitude but mutually inverted phasesinterfere, it is possible to reduce the light intensity of the reflectedlight to zero. In addition, when the amplitudes of the lights differ,the light intensity of the reflected light differs depending on theamplitudes.

Therefore, it is possible to form bright and dark patterns on an imageformation surface by using the photomask which has the concavo-convexreflecting surface.

Next, a more detailed description will be given of the embodiment of thereflection type photolithography method according to the presentinvention. When the reflected light 11 is incident perpendicularly tothe reflecting surface of the photomask 1, it is possible to generatethe lights having a phase difference of approximately λ/2 (π radians) byforming the concavo-convex pattern having a height difference of λ/4,where λ denotes the wavelength of the reflected light 11. In FIG. l, thereflected light 11 is shown incident to the reflecting surface of thephotomask 1 with a certain incident angle which is not perpendicular tothe reflecting surface so as to facilitate the understanding of thepropagation of the reflected lights. It is, however, readily apparentthat the height difference of the concavo-convex pattern may be set toobtain the desired phase difference for each incident angle used. It isalso possible to generate lights having phase differences of 0, π/2, π,3π/4 and the like by providing the concavo-convex pattern with differentheights. When the relative concave first phase region 2 and the relativeconvex second phase region 3 have the same reflectivity, the lightreflected from the first phase region 2 and the light reflected from thesecond phase region 3 have the same light amplitude.

As described above, the reflected light 12 including the lights havingthe phase difference is imaged on the photoresist layer 8 on thesubstrate 4 using the imaging mirror 5. When the reflected light 12 isimaged on the photoresist layer 8, the light reflected from the firstphase region 2 and the light reflected from the adjacent second phaseregion 3 mutually diffract and partially overlap. As a result, lightinterference occurs and a pattern of the light intensity distribution isgenerated. In other words, the pattern of the light intensitydistribution is generated based on the amplitude and phase of thereflected lights. Accordingly, it is possible to obtain ablack-and-white pattern when the photoresist layer 8s developed using apredetermined exposure level as a threshold value. For example, when apositive resist is used for the photoresist layer 8, it is possible toleave as a pattern only a region in which the light intensity fallsbelow a predetermined level.

A heavy hydrogen (or deuterium) lamp, and excimer laser or the like maybe used as the light source 7. Particularly, an ArF excimer laser whichemits a laser beam having a wavelength of approximately 193 nm or a F₂excimer laser which emits a laser beam having a wavelength ofapproximately 157 nm is suited as the light source 7. The mirrors 5 and6 may be formed by polishing a glass substrate into a mirror surface anddepositing an Al layer on the mirror surface. Since no chromaticaberration is introduced in the reflection optical system, it is alsopossible to use light which does not have a single wavelength. Forexample, it is possible to maintain and utilize the effectiveinterference within a wavelength region of approximately 10% of thewavelengths.

Next, a description will be given of a first embodiment of a photomaskaccording to the present invention, by referring to FIGS. 3A and 3B.FIG. 3A shows a cross section of the first embodiment of the photomask,and FIG. 3B shows an intensity distribution of reflected light.

In this embodiment, the reflection type photomask 1 has a reflectingsurface formed on a top surface thereof. The reflecting surface includesa first phase region 2 which is arranged on a specific plane, and asecond phase region 3 which is arranged on a plane which isapproximately λ/4 higher than the specific plane, where λ denotes thewavelength of the light used. Because the height difference of the firstand second phase regions 2 and 3 is approximately λ/4, the lightreflected by the first phase region 2 has a phase which is delayed by(λ/4)×2=λ/2 compared to the phase of the light reflected by the secondphase region 3, assuming that the light is incident approximatelyperpendicularly to the reflecting surface of the photomask 1. That is,the lights reflected by the first and second phase regions 2 and 3 havea phase difference π.

FIG. 3B shows the reflected light intensity distribution which isobtained when the reflected light from the photomask 1 shown in FIG. 3Ais imaged on the photoresist layer 8. The lights having the phasedifference π are mixed at a boundary of the first and second phaseregions 2 and 3, and as a result, a position where the reflected lightintensity becomes zero is introduced. Hence, by using the lights havingthe same amplitude but mutually inverted phases, it becomes possible toform an extremely narrow black pattern, that is, a region in which thelight intensity is zero or extremely small. Such a black pattern remainsafter developing the photoresist layer 8 which is made of a positiveresist.

Next, a description will be given of methods of forming the secondembodiment of the photomask according to the present invention, byreferring to FIGS. 4A and 4B.

FIG. 4A shows a case where a substrate of the photomask 1 is made of ametal having a high reflectivity with respect to the wavelength of thelight used, and a reflecting surface 15 is formed by polishing a topsurface of the metal substrate to a mirror finish. This reflectingsurface 15 is covered by a photoresist layer or the like and a maskpattern is formed. Then, an etching is made to etch the mirror surface15 to a depth of approximately λ/4 so as to form the first phase region2, and the mask pattern is thereafter removed. As a result, it ispossible to form the concavo-convex pattern on the reflecting surface15, that is, the first and second phase regions 2 and 3. Measures mustbe taken so that the etched surface has a mirror finish and the firstand second phase regions 2 and 3 have approximately the samereflectivity.

FIG. 4B shows a case where the substrate of the photomask 1 is made of acrystal having a top surface which is polished to a mirror finish. Theconcavo-convex pattern is formed on the crystal substrate, similarly asin the case of the metal substrate shown in FIG. 4A. Thereafter, a metalreflecting layer 20 having a high reflectivity is deposited on theconcavo-convex surface of the crystal substrate by a vapor deposition,plating or the like so as to form the reflecting surface 15. As aresult, the first and second phase regions 2 and 3 are formed on themetal reflecting layer 20.

Next, a description will be given of a second embodiment of thephotomask according to the present invention, by referring to FIGS. 5Aand 5B. FIG. 5A shows a cross section of the second embodiment of thephotomask, and FIG. 5B shows an intensity distribution of reflectedlight.

In this embodiment, the photomask 1 has a narrow groove 16 formed in thereflecting surface 15. The width of the groove 16 is set within a rangesuch that the light intensity change caused by the interference at sideedges 17 on both sides of the groove 16 mutually overlap on the imagingplane. In other words, the width of the groove 16 is set so that twoblack patterns on the imaging plane corresponding to the two side edges17 overlap and may only be recognized as a single black pattern.Accordingly, the light intensity does not become large in a region inwhich the light reflected by the bottom surface of the groove 16 isimaged, and a black pattern having a low light intensity is formed at aposition corresponding to the groove 16.

Next, a description will be given of a third embodiment of the photomaskaccording to the present invention, by referring to FIGS. 6A and 6B.FIG. 6A shows a cross section of the third embodiment of the photomask,and FIG. 6B shows an intensity distribution of reflected light.

In this embodiment, the photomask 1 has a narrow projection 18 formed onthe reflecting surface 15. The width of the projection 18 is set withina range such that the light intensity change caused by the interferenceat side edges 17 on both sides of the projection 18 mutually overlap onthe imaging plane. In other words, the width of the projection 18 is setso that two black patterns on the imaging plane corresponding to the twoside edges 17 overlap and may only be recognized as a single blackpattern. Accordingly, the light intensity does not become large in aregion in which the light reflected by the top surface of the projection18 is imaged, and a black pattern having a low light intensity is formedat a position corresponding to the projection 18.

Of course, it is possible to combine the structures shown in FIGS. 5Aand 6A. In other words, it is possible to form a groove having a depthof approximately λ/4 from a reference surface and a projection having aheight of approximately λ/4 from the reference surface.

In addition, the lights reflected from the reflecting surface 15 of thephotomask 1 need not necessarily have the phase difference π. In otherwords, the groove or projection may be formed on the reflecting surface15 so that the reflected lights have a phase difference other than πsuch as π/2 and 3π/4. Such an intermediate phase difference isespecially suited for forming an open figure when forming a blackpattern at a boundary of a region in which the phase difference is π asshown in FIGS. 3A and 3B.

Next, a description will be given of a fourth embodiment of thephotomask according to the present invention, by referring to FIGS. 7Aand 7B. FIG. 7A shows a cross section of the fourth embodiment of thephotomask, and FIG. 7B shows an intensity distribution of reflectedlight.

In this embodiment, a substrate 25 of the photomask 1 is made of amaterial such as crystal and transmits light. A concavo-convex patternis formed on a top surface of the substrate 25. A reflecting layer 20 isformed partially on the concavo-convex pattern to form the reflectingsurface 15. A wide concave region 21 is formed in the reflecting surface15, and the reflecting layer 20 is formed on the reflecting surface 15at only a peripheral part of the concave region 21 and the surface ofthe substrate 25 is exposed at a central part of the concave region 21.In other words, the central part of the concave region 21 forms anon-reflecting region. The width of the reflecting layer 20 at theperipheral part of the concave region 21 is limited to a width such thata white pattern having a large light intensity is not formed on theimage forming plane. When the light is incident to the reflectingsurface 15 shown in FIG. 7A, the light is strongly reflected in theregion in which the reflecting layer 20 is provided and is substantiallynot reflected in the region in which no reflecting layer 20 is provided.

As shown in FIG. 7B, the reflected light intensity decreases in a regionwhich corresponds to a narrow groove 22 shown in FIG. 7A. In addition,the reflected light intensity decreases in a region which corresponds tothe concave region 21 which has the relatively large area because thereflecting layer 20 is formed only in the peripheral part of the concaveregion 21 and the reflected lights of mutually inverted phasesinterfere. Since the reflecting layer 20 does not exist on the innerside of the peripheral part of the concave region 21, no reflected lightexists and the reflected light intensity is maintained low.

Accordingly, a non-reflecting surface is formed to prevent thegeneration of the reflected light itself for a region which is widerthan a predetermined width when forming a relatively wide black pattern,that is, a pattern having a low reflected light intensity.

Next, a description will be given of a fifth embodiment of the photomaskaccording to the present invention, by referring to FIGS. 8A and 8B.FIG. 8A shows a cross section of the fifth embodiment of the photomask,and FIG. 8B shows an intensity distribution of reflected light.

In this embodiment, the substrate 25 of the photomask 1 is made of amaterial such as crystal and transmits light. A concavo-convex patternis formed on a top surface of the substrate 25. The reflecting layer 20is formed partially on the concavo-convex pattern to form the reflectingsurface 15. A wide convex region 24 is formed on the reflecting surface15 at a position corresponding to that of the wide concave region 21shown in FIG. 7A, and the reflecting layer 20 is formed on thereflecting surface 15 at only a peripheral part of the convex region 24and the surface of the substrate 25 is exposed at a central part of theconvex region 24. In other words, the central part of the convex region24 forms a non-reflecting region. The width of the reflecting layer 20at the peripheral part of the convex region 24 is limited to a widthsuch that a white pattern having a large light intensity is not formedon the image forming plane. When the light is incident to the reflectingsurface 15 shown in FIG. 8A the light is strongly reflected in theregion in which the reflecting layer 20 is provided and is substantiallynot reflected in the region in which no reflecting layer 20 is provided.

As shown in FIG. 8B, the reflected light intensity decreases in a regionwhich corresponds to a narrow projection 23 shown in FIG. 8A. Inaddition, the reflected light intensity decreases in a region whichcorresponds to the convex region 24 which has the relatively large areabecause the reflecting layer 20 is formed only in the peripheral part ofthe convex region 24 and the reflected lights of mutually invertedphases interfere. Since the reflecting layer 20 does not exist on theinner side of the peripheral part of the convex region 24, no reflectedlight exists and the reflected light intensity is maintained low.

Accordingly, a non-reflecting surface is formed to prevent thegeneration of the reflected light itself for a region which is widerthan a predetermined width when forming a relatively wide black pattern,that is, a pattern having a low reflected light intensity.

Therefore, the photomask 1 shown in FIG. 7A and the photomask 1 shown inFIG. 8A have essentially the same functions. Of course, it is possibleto combine the structures of FIGS. 7A and 8A.

In the embodiments shown in FIGS. 7A and 8A, the non-reflecting surfaceis formed by exposing the substrate surface. However, when the substrate25 is made of a transparent material, the substrate surface usually hasa certain reflectivity and a slight reflection occurs. Hence, it ispossible to reduce the reflection at the substrate surface by forming areflection preventing layer.

Next, a description will be given of sixth and seventh embodiments ofthe photomask according to the present invention, by referring to FIGS.9A and 9B. FIG. 9A shows a cross section of the sixth embodiment of thephotomask, and FIG. 9B shows a cross section of the seventh embodimentof the photomask. In FIGS. 9A and 9B, those parts which are the same asthose corresponding parts in FIGS. 7A and 8A are designated by the samereference numerals, and a description thereof will be omitted.

In the sixth embodiment shown in FIG. 9A, the substrate 25 of thephotomask 1 is made of a material which is transparent with respect tothe wavelength of the light which is used. A concavo-convex pattern isformed on a top surface of the substrate 25. The depth of the concaveregion or the height of the convex region is λ/4. A reflectionpreventing layer 26 having a thickness λ/4 is formed on theconcavo-convex pattern. The reflection preventing layer 26 is made of amaterial having an intermediate refractive index between the refractiveindex of air or vacuum. The refractive index of the material used forthe reflection preventing layer 26 is desirably close to a valueX^(1/2), where X denotes the refractive index of the substrate 25. Forexample, when the substrate 25 is made of crystal, the reflectionpreventing layer 26 is made of MgF₂.

The reflection preventing layer 26 may have a multi-layer structurehaving three, five or more layers, for example. In this case, therefractive index of each layer of the multi-layer structure is selectedso as to minimize the reflection from the reflection preventing layer26.

The reflecting layer 20 is formed partially on the concavo-convexpattern of the reflection preventing layer 26 to form the reflectingsurface 15. For example, the reflecting layer 20 is made of a metalhaving a high reflectivity. The wide concave region 21 is formed in thereflecting surface 15, and the reflecting layer 20 is formed on thereflecting surface 15 at only a peripheral part of the concave region 21and the surface of the reflection preventing layer 26 is exposed at acentral part of the concave region 21. For example, the reflecting layer20 may be formed on the entire surface of the reflection preventinglayer 26 and the reflecting layer 20 at the central part of the concaveregion 21 may be removed by an etching. In other words, the central partof the concave region 21 forms a non-reflecting region.

The seventh embodiment shown in FIG. 9B is basically the same as thesixth embodiment shown in FIG. 9A, except that the convex region 24 isprovided in place of the concave region 21.

According to the sixth and seventh embodiments, the reflected light fromthe substrate 25 and the reflected light from the surface of thereflection preventing layer 26 cancel each other, thereby reducing thereflected light.

Next, a description will be given of a first pattern of a semiconductordevice formed by the reflection type photomask according to the presentinvention, by referring to FIGS. 10A through 10C. FIGS. 10A and 10Brespectively show the plan view and cross sectional view of thephotomask, and FIG. 10C shows the plan view of the first pattern whichis formed.

In FIGS. 10A and 10B, a rectangular projection 31 is formed on a surfaceof a crystal substrate 33. A reflecting layer 32 made of Al covers theprojection 31 and a peripheral region of the projection 31. Theperipheral region has a predetermined width.

When the photomask shown in FIGS. 10A and 10B is used to develop aphotoresist layer using a reflected light, the first pattern shown inFIG. 10C is obtained. This first pattern is a contact hole. Theintensity of the reflected light obtained from the reflecting layer 32at the central part of the projection 31 is large, but the intensity ofthe reflected light from the reflecting layer 32 at other parts is lowbecause the reflected light from the peripheral part of the projection31 and the the reflected light from the reflecting layer 32 on thesubstrate surface mutually interfere. For this reason, a rectangularwhite pattern 35, that is, a region in which the reflected lightintensity is large), is obtained at a part corresponding to the centralpart of the projection 31. On the other hand, a black pattern, that is,a region in which the reflected light intensity is low, is obtained atthe peripheral part of the projection 31 because reflected light doesexist but the intensity of the reflected light is low due to the mutualinterference. In addition, a black pattern is also obtained at a partcorresponding to the part where the substrate surface is exposed becausethe reflected light intensity itself is low at this part.

Next, a description will be given of a second pattern of a semiconductordevice formed by the reflection type photomask according to the presentinvention, by referring to FIGS. 11A through 11C. FIGS. 11A and 11Brespectively show the plan view and cross sectional view of thephotomask, and FIG. 11C shows the plan view of the second pattern whichis formed.

In FIGS. 11A and 11B, a rectangular projection 36 is formed on a surfaceof a crystal substrate 38. A reflecting layer 37 made of Al covers aperipheral part of the projection 36 and the substrate surface. Thesubstrate surface is exposed at only a central part of the projection36.

When the photomask shown in FIGS. 11A and 11B is used to develop aphotoresist layer using a reflected light, the second pattern shown inFIG. 11C is obtained. This second pattern is a metal interconnectionpattern. At a narrow pattern 39a or 39b shown in FIG. 11A, the reflectedlight from the lower surface of the reflecting layer 37 on both sides ofthe narrow pattern 39a or 39b and the reflected light from the highersurface of the reflecting layer 37 interfere, thereby decreasing thereflected light intensity and forming one black pattern. In addition, ablack pattern is similarly formed at the peripheral part of theprojection 36 due to interference, and a black pattern is formed at thecentral part of the projection 36 since the substrate surface is exposedand the reflected light itself is reduced at the central part.

As may be understood from the embodiments described above, the presentinvention is particularly effective for the photolithography using a farultraviolet light which has a wavelength of 200 nm or less. However, itis of course possible to employ the present invention for thephotolithography using a light which has a longer wavelength.

A transmission type photomask utilizing the phase shift of transmittedlights was previously proposed in a U.S. patent application Ser. No.516,347 filed Apr. 27, 1990. FIG. 12A shows a cross section of theproposed photomask having a transparent substrate 100 and a phase shiftlayer 101. In order to obtain the phase shift π between the lighttransmitted through only the substrate 100 and the light transmittedthrough the substrate 100 and the phase shift layer 101 for an i-ray,for example, the phase shift layer 101 must have a thickness of 390 nm.However, when the phase shift layer 101 is relatively thick, unwantedreflections occur at an edge part 103 of the phase shift layer 101 andit becomes difficult to form fine and accurate patterns.

On the other hand, since the present invention utilizes the photomaskshown in FIG. 12B having a substrate 200 having a concavo-convexreflecting surface, the height difference between the concave region andthe convex region of the reflecting surface 200 only needs to be 91 nmin order to obtain the phase shift π between the light reflected at theconcave region and the light reflected at the convex region for thei-ray. Hence, compared to the transmission type photomask shown in FIG.12A, unwanted reflections which occur at an edge part 203 of thesubstrate 200 virtually negligible and it becomes possible to form fineand accurate patterns. Therefore, when the light having a wavelength of180 nm or less is used, it possible to accurately form a pattern whichhas a size of 0.2 μm or less.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A photolithography method comprising the stepsof:irradiating a light from a light source on a reflection typephotomask which has a concavo-convex reflecting surface which includes afirst region and a second region which have a relative height differenceso that a light reflected from the first region and a light reflectedfrom the second region have a predetermined phase difference caused by adifference in lengths of optical paths of the lights which reflect atthe first and second regions; imaging the lights reflected from thefirst and second regions of the photomask onto a photoresist layer whichis formed on a wafer by use of an optical system so as to develop apattern on the photoresist layer; and developing the pattern on thephotoresist layer depending on a light intensity of the light imagedthereon.
 2. The photolithography method as claimed in claim 1, whereinthe light from the light source is obtained via a mirror which reflectsthe light emitted from the light source, and the optical system includesa mirror which reflects and images the lights reflected from the firstand second regions of the photomask onto the photoresist layer.
 3. Thephotolithography method as claimed in claim 1, wherein the photomaskincludes a substrate which has a concavo-convex substrate surface, and areflecting layer which is formed on the substrate surface and has asurface which forms the reflecting surface.
 4. The photolithographymethod as claimed in claim 3, wherein the substrate is made of amaterial which is transparent with respect to the light which isirradiated on the reflecting surface.
 5. The photolithography method asclaimed in claim 4, wherein the first region includes a concave partformed in the substrate, and the reflecting layer is formed on thesubstrate surface excluding a central part of the concave part.
 6. Thephotolithography method as claimed in claim 4, wherein the second regionincludes a convex part formed on the substrate, and the reflecting layeris formed on the substrate surface excluding a central part of theconvex part.
 7. The photolithography method as claimed in claim 1,wherein at least one of the first and second regions has such a widththat an interference pattern generated by a light reflected at one endof the first and/or second region approximately overlaps an interferencepattern generated by a light reflected at the other end of the firstand/or second region on the photoresist layer.
 8. The photolithographymethod as claimed in claim 1, wherein the photomask includes a substratewhich has a concavo-convex substrate surface, a reflection preventinglayer which is formed on the substrate surface and a reflecting layerwhich is formed on at least a part of the reflection preventing layerand has a surface which forms the reflecting surface.
 9. Thephotolithography method as claimed in claim 8, wherein the substrate ismade of a material which is transparent with respect to the light whichis irradiated on said reflecting surface.
 10. The photolithographymethod as claimed in claim 9, wherein the first region includes aconcave part formed in the substrate, and the reflecting layer is formedon the reflection preventing layer excluding a central part of theconcave part.
 11. The photolithography method as claimed in claim 9,wherein the second region includes a convex part formed on thesubstrate, and the reflecting layer is formed on the reflectionpreventing layer excluding a central part of the convex part.
 12. Thephotolithography method as claimed in claim 1, wherein the photomaskfurther includes a third region formed in a central part of one of thefirst and second regions, said first and second regions having a largereflectivity compared to a reflectivity of said third region.
 13. Thephotolithography method as claimed in claim 1, wherein the photomaskincludes a substrate which is made of a metal and has a concavo-convexsubstrate surface, and said reflecting surface is the substrate surface.14. A photolithography method comprising the steps of:irradiating alight from a light source on a reflection type photomask which has asubstrate having a concavo-convex substrate surface and a reflectingsurface formed on at least part of the concavo-convex substrate surfaceand having a concavo-convex reflecting surface which includes a firstregion and a second region which have a relative height difference sothat a light reflected from the first region and a light reflected fromthe second region have a predetermined phase difference caused by adifference in lengths of optical paths of the lights which reflect atthe first and second regions; imaging the lights reflected from thefirst and second regions of the photomask onto a photoresist layer whichis formed on a wafer by use of an optical system so as to develop apattern on the photoresist layer; and developing the pattern on thephotoresist layer depending on a light intensity of the light imagedthereon.